Polytetrafluoroethylene reins (hereafter referred to as "PTFE") are widely used in various applications as a result of their superior electrical characteristics, heat resistance and chemical resistance. In cases where such resins are used electrical insulating materials, attempts have been made to further improve the electrical characteristics by making said resins porous.
Since molten PTFE has a markedly high viscosity, foaming methods used in ordinary thermoplastic resins or other fluororesins (e.g., physical foaming accomplished by blowing in an inert gas, or chemicl foaming accomplished by means of foaming agents, etc.) cannot be used to foam PTFE. Accordingly, special methods must be used in order to manufacture porous PTFE. One method is a method in which (i) substances which can be removed by extraction or dissolution are mixed with PTFE, the mixture is molded under pressure, and the aforementioned substances are then removed. Another method is one in which a liquid lubricant is added to finely powdered PTFE, this mixture is molded under conditions in which a shearing force is applied by extrusion or rolling, the aforementioned liquid lubricant is then removed, and the material is drawn and sintered. Still another is one method in which an unsintered PTFE molding is drawn in a liquid which can wet PTFE (e.g., a halogenated hydrocarbon, petroleum hydrocarbon, alcohol or ketone, etc.), and is then sintered.
However, in all of these methods, the porous material obtained is a continuously porous material. As a result, the internal pores are easily crushed by even a slight compressive force, so that portions of the material that are subjected to compression tend to be converted into a non-porous structure. This tendency is especially conspicuous in cases where the porosity is increased in order to lower the dielectric constant. Accordingly, in cases where such materials are formed into tapes or sheets, etc., and used as insulators for electric wires or printed circuit boards, etc., the electrical characteristics of the materials (dielectric constant, etc.) tend to be unstable, so that said materials are extremely difficult to handle.